Blender 2.8 Material colors bleeding through faces in render
From what I can see from the screenshot, it looks like you've duplicated the faces and left them in the same place as the source face you duplicated it from.

This will cause problems because the renderer wont know which one is on top of the other - and so you end up with all those jaggy artefacts where it's kind of struggling with which one should be rendered over the other.

Quick solve to that is to just move the black faces out - can be by the smallest amount, just so they don't occupy exactly the same coordinates.

But, if the aim was to make those faces black, then duplicating the faces on top isn't the best method.

So I presume you have that mesh with one 'red' material ye?
Add a new material slot, create a black material in that, then go to edit mode of the mesh (if it's all one mesh) and select the face you want black, then 'assign' the black material to that face. (obviously get rid of your duplicate faces too)

If that's not the case, and the mesh just has duplicate faces, then you need to do some cleaning and remove the duplicate faces, merge vertices etc.